Mammalian lymphocytes are considered to include populations of long- and short-lived cells. While no comparable studies have been published for the chicken, a preliminary experiment in our laboratory has suggested the presence of such populations of lymphocytes in the chicken's thymus. The initial objective of this proposal will be examination of the thymus, bursa, and bone marrow for long- and short-lived lymphocytes by employing scintillation counting and autoradiography. The contribution of mammalian T and B cells to the pool of long- and short-lived lymphocytes is still controversial. Therefore, a second objective will be to determine the contribution of the B cell to the population of long- and short-lived lymphocytes. The design of the second objective, which includes embryonic elimination of the bursa with testosterone propionate (TP), will allow us to introduce a third objective: the response of embryonic thymic T cells and bone marrow lymphocytes to TP.